Eye registration and astigmatism alignment control systems and method

ABSTRACT

An orientation system for corrective eye surgery includes a camera for performing a first image mapping of a patient&#39;s eye using a predetermined eye feature and software for processing the first image map to determine an edge location of the feature. A second image mapping of the eye is performed with the patient in a different position. The second image map is processed to locate the predetermined eye feature. Correlation of the mappings is used to calculate an orientational change of the eye between the two positions. The data are used to calculate an adjustment to be applied to a corrective prescription for application by the surgical procedure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/838,665,filed Apr. 19, 2001, now U.S. Pat. No. 6,702,806, which itself claimspriority from and incorporates by reference commonly owned provisionalapplications Ser. No. 60/198,393, filed Apr. 19, 2000, “AstigmatismAlignment Control Device and Method,” and Ser. No. 60/270,071, filedFeb. 20, 2001, “Eye Registration Apparatus and Method.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to systems and methods for improvingobjective measurements preceding corrective eye surgery, and, moreparticularly, to such systems and methods for improving results ofcorrective laser surgery on the eye.

2. Description of Related Art

Laser-in-situ-keratomileusis (LASIK) is a common type of laser visioncorrection method. It has proven to be an extremely effective outpatientprocedure for a wide range of vision correction prescriptions. The useof an excimer laser allows for a high degree of precision andpredictability in shaping the cornea of the eye. Prior to the LASIKprocedure, measurements of the eye are made to determine the amount ofcorneal material to be removed from various locations on the cornealsurface so that the excimer laser can be calibrated and guided forproviding the corrective prescription previously determined by themeasurements. Refractive laser surgery for the correction of astigmatismtypically requires that a cylindrical or quasicylindrical ablationprofile be applied to the eye. The long axis of this profile must beproperly oriented on the eye in order to accurately correct the visualaberration.

An objective measurement of a patient's eye is typically made with thepatient sitting in an upright position while focusing on a target image.A wavefront analyzer then objectively determines an appropriatewavefront correction for reshaping the cornea for the orientation of theeye being examined. The LASIK or PRK procedure is then typicallyperformed with the patient in a prone position with the eye lookingupward.

It is well known that the eye undergoes movement within the socketcomprising translation and rotation (“cyclotorsion”) as the patient ismoved from the upright measuring position to the prone surgery position.Techniques known in the art for accommodating this movement haveincluded marking the eye by cauterizing reference points on the eyeusing a cautery instrument (U.S. Pat. No. 4,476,862) or causticsubstance, a very uncomfortable procedure for the patient. It is alsoknown to mark a cornea using a plurality of blades (U.S. Pat. No.4,739,761). The injection of a dye or ink is also used to mark thereference locations to identify the orientation of the eye duringmeasurement, permitting a positioning of the corrective profile to thesame orientation prior to surgery. However, the time delay frommeasurement to surgery often causes the ink to run, affecting theaccuracy of an alignment. Making an impression on the eye (U.S. Pat. No.4,705,035) avoids the caustic effects of cauterizing and the runningeffect of the ink. However, the impression loses its definition quicklyrelative to the time period between the measurement and surgery.

For correction of astigmatism, it is known to mark the corneapreparatory to making the surgical incisions (U.S. Pat. No. 5,531,753).

Tracker systems used during the surgical procedure or simply forfollowing eye movement, while the patient is in a defined position, areknown to receive eye movement data from a mark on a cornea made using alaser beam prior to surgery (U.S. Pat. No. 4,848,340) or fromilluminating and capturing data on a feature in or on the eye, such as aretina or limbus (U.S. Pat. Nos. 5,029,220; 5,098,426; 5,196,873;5,345,281; 5,485,404; 5,568,208; 5,620,436; 5,638,176; 5,645,550;5,865,832; 5,892,569; 5,923,399; 5,943,117; 5,966,197; 6,000,799;6,027,216).

SUMMARY OF THE INVENTION

A system and method are provided for accurately orienting an eye forsurgery to the same orientation it had during an objective measurement.An orientation correction algorithm is provided to the software drivinga corrective surgical device. Further, pairs of eye images taken atdifferent times can be aligned (registered). The system and method alsoavoids placing a patient in an uncomfortable or harmful situation.

A first embodiment of the system of the present invention comprisesmeans for performing a first image mapping of an eye of a patientsituated in a first position using a predetermined eye feature. Meansare further provided for performing a second image mapping of the eye ofthe patient in a second position different from the first position usingthe predetermined eye feature. Means are also provided for processingthe first and the second image map to determine an edge location of thefeature in two dimensions and to locate the predetermined eye feature.Finally, software means are included for calculating an orientationalchange to be applied to a corrective prescription for a surgicalprocedure to be performed on the eye with the patient in the secondposition. The procedure may comprise, for example, implementing acorrection profile that had been determined with the patient in thefirst position with, for example, a wavefront analysis and conversionsystem for calculating an ablation profile for a cornea, such asdescribed in co-pending and co-owned U.S. patent application Ser. No.09/566,668, the disclosure of which is hereby incorporated by reference.

The method of this first embodiment of the present invention is fororienting a corrective program for eye surgery and comprises the stepsof performing a first image mapping of an eye of a patient in a firstposition using a predetermined eye feature. The method also comprisesthe steps of performing a second image mapping of the eye of the patientin a second position different from the first position using the featureand processing the first and the second image map to determine an edgelocation of the feature in two dimensions and to locate the feature.Next an orientational change to be applied to a corrective prescriptionfor a surgical procedure to be performed on the eye with the patient inthe second position is calculated. The procedure comprises a correctionprofile determined with the patient in the first position.

Thus this aspect of the present invention provides a system and methodfor achieving a precise registration of the eye with a measurement ofthe movement of an eye feature. As a result, the prescriptionmeasurement for reshaping the cornea will account for the rotation andtranslation of the eye occurring between measurements made with thepatient in a sitting position and laser surgery with the patient in aprone position.

Yet another embodiment of the orientation system for eye surgery forcorrecting astigmatism comprises means for making two alignment marks onan eye of a patient with the patient in a first position. Means are alsoprovided for imaging the eye with the patient in a second position thatis different from the first position. The system also comprises acomputer that has input and output means. The input means are inelectronic connection with the imaging means, and an operator inputdevice is in electronic communication with the computer input means.Means for displaying the eye image to an operator are also incommunication with the computer input and output means.

First software means are resident in the computer for superimposing agraphical reticle means onto the eye image on the displaying means andfor permitting the graphical reticle means to be moved by the operatorunder control of the operator input means. The reticle means comprise aline for aligning with the two alignment marks. Second software meansalso resident in the computer are for calculating an orientationalchange to be applied to a corrective surgical procedure to be performedon the eye with the patient in the second position. As above, theprocedure comprises a correction profile determined with the patient inthe first position.

The features that characterize the invention, both as to organizationand method of operation, together with further objects and advantagesthereof, will be better understood from the following description usedin conjunction with the accompanying drawings. It is to be expresslyunderstood that the drawings are for the purpose of illustration anddescription and are not intended as a definition of the limits of theinvention. These and other objects attained, and advantages offered, bythe present invention will become more fully apparent as the descriptionthat now follows is read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the system of a first embodiment of thepresent invention.

FIG. 2 is a block diagram of the data flow.

FIG. 3 is a view of the original image, before image processing, withfeature boxes around the features to be used as registration regions.

FIG. 4 is a view of a Gauss-filtered intensity profile with θ₁=0,showing the edge in an x direction.

FIG. 5 is a view of a Gauss-filtered intensity profile with θ₂=Π/2,showing the edge in a y direction.

FIG. 6 is a view of a geometric average of FIGS. 4 and 5.

FIG. 7 is a view with threshold application.

FIG. 8 is a view of the image following application of the thinfunction.

FIG. 9 is a schematic diagram of the system of a second embodiment ofthe present invention.

FIG. 10 is a representation of an image of an eye as viewed on agraphical user interface in the second embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A description of the preferred embodiments of the present invention willnow be presented with reference to FIGS. 1-10.

A schematic diagram of a system 10 of a first embodiment of theinvention is shown in FIG. 1, data flow and resulting images in FIG. 2,and original and processed images in FIGS. 3-8. A section on the imageprocessing algorithms embodied herein follows the description of thesystem and method. In an exemplary embodiment of the system 10, apatient's eye 11 is image mapped in a substantially upright position bycapturing a first video image 12 using a camera such as acharge-coupled-device (CCD) camera 13. Such an image 12 is illustratedin FIG. 3. The first image 12 is stored in a database 14 in electroniccommunication with a computer 15 and labeled as an original image from afirst measurement.

Next an objective measurement is made on the eye 11 to determine adesired correction profile, using a measurement system 16 such as thatdisclosed in copending application Ser. No. 09/566,668, although this isnot intended as a limitation.

Once the correction profile is determined, the patient is made ready forsurgery, and placed in the second position, which is typically prone.Alternatively, the first scan to determine the correction profile may bemade in a different location and at a time prior to the surgicalprocedure, the time interval being, for example, several weeks. Then asecond image map 17 is collected using a second camera 18, incommunication with a second system 38 for performing surgery, and thesedata are also stored in the database 14. In a preferred embodiment boththe first 13 and the second 18 cameras are adapted to collect colorimages, and these images are then converted using software resident onthe computer 15 to intensity profiles 19,20 as grey-scale images.Alternatively, color images may be used. It is useful to collect colorimages for viewing by the physician, since image mapping of the eye 11may be made using preselected identifiable images such as blood vessels21,22 typically seen within the sclera 23. In a color image, the redcolor of the vessels 21,22 is clearly identifiable. Typically the secondimage map 17 is collected during setup prior to surgery using acorrection system such as is disclosed in application Ser. No.09/566,668, although this is not intended as a limitation. As the imagemaps 12,17 are typically collected with different cameras 13,18, thequalities of the images 12,17 are expected to be different, making theimage processing steps of great importance.

Next the intensity profile 19 of the first video image 12 is processedthrough a weighting function such as a filter, in a preferred embodimenta Gauss filter, although this is not intended as a limitation. Thisfilter is for eliminating noise within the intensity profiles fordefining image edge locations in both an x and a y orientation toprovide two-dimensional information, and also to emphasize or highlightfeatures for subsequent processing. The Gauss filter establishes a firstmodified intensity profile 24 with θ₁=0, as an example, as shown in FIG.4, an edge view in the x direction, the θ values taken relative to the xaxis. The Gauss filter is again applied to the intensity profiles toestablish a second modified intensity profile 25, with θ₂=Π/2, as shownin FIG. 5, an edge view in a y direction.

A geometric average of the filtered x and y orientations is performedand processed to eliminate unwanted noise levels to form a firstfiltered intensity profile 26 for the first image 12, yielding a view asshown in FIG. 6. This first filtered intensity profile 26 has beencalculated by taking the square root of the sum of the squares of thefirst 24 and the second 25 modified intensity profiles.

The above process is repeated for the second image 17, to produce, fromthe second intensity profile 20, a third modified intensity profile 27from application of a Gauss filter with θ₃=0 and a fourth modifiedintensity profile 28, with θ₄=Π/2, and geometric averaging to produce asecond filtered intensity profile 29.

Next an adaptive signal threshold is selected to reduce background noisefor the first 26 and the second 29 filtered intensity profiles,resulting in first 30 and second 31 thresholded images, with the firstthresholded image 30 shown in FIG. 7. The λ may be different for thefirst 26 and the second 29 filtered intensity profiles; here λ=0.03.

The first 26 and the second 29 filtered intensity profiles 26,29 arethen processed through a “thin function” to produce a first 32 and asecond 33 edge image, with the first edge image 32 shown in FIG. 8. Thisstep completes the image processing. Next the surgeon selects one ormore features in the eye 11, shown as a first 21 and a second 22 feature(here, blood vessels) in FIG. 3. The selected features are used forcorrelating between filtered images for the second (surgical) positionof the eye 11 with that of the first (measurement) position. Otherfeatures may also be used if sufficiently prominent blood vessels arenot present. The excimer laser 36 coordinates are then reoriented toaccommodate the rotation and translation that took place when moving thepatient from a measurement instrument to the surgical device.

The operator proceeds to locate the limbus 34 using a graphical userinterface (GUI) while viewing the still image of the eye (FIG. 3). Byway of example, a reticle 37 is moved in position to coincide with thelimbus 34. The reticle size may be changed, including a diameter of acircular reticle, or optionally both minor and major radius of anelliptical reticle. The operator then selects a feature or features21,22 of the eye 11 to be used by outlining the selected feature orfeatures 21,22 within graphical “feature boxes,” and the above processis automatically performed by the “push of a button,” which takes onlyseconds to complete in the exemplary embodiment.

Using the first 32 and second 33 edge images, and knowing the center ofthe reticle 37 (circle or ellipse), the computer 15 determinescoordinates for the selected features 21,22.

Image mapping within each feature 21,22 box is a process of using thetransformation described below. By way of example, the process fixes thefirst edge image 32 and varies the angle of orientation for second edgeimage 33.

The computer 15 overlays the first 32 and the second 33 edge image withregard to center and compares each point within the feature 21,22 box.Each edge image 32,33 is also compared for different values of θ todetermine maximum matching points. The computer 15 moves the centerrelation for the edge images 32,33 and seeks to improve its location(center a, b) and value for a θ orientation. Each feature box or area(pixels within area) is processed before moving the center and iscompleted for every θ (typically −25°≦θ≦+25°), which will typicallycover a patient's eye rotation when moving from an upright to a proneposition. Completing the entire process takes less than 30 sec.

The treatment pattern, typically a laser shot pattern, is thus modifiedto account for eye rotation resulting from the patient's movement fromupright to prone position. In addition, an eye tracking feature of thesecond system 38 can account for eye movement during surgeryindependently of the camera 18.

By way of further example, code for carrying out the process steps toobtain the image of FIG. 8, and code for carrying out an exemplaryembodiment of the above for the steps including the feature coordinatedetermination through the processing of the feature boxes, are disclosedin provisional application 60/270,071, which is hereby incorporatedherein by reference.

The present invention also provides a system and method for aligning(registering) pairs of eye images taken at different times. By way ofexample, images may be taken at:

-   -   1. An undilated pupil at centration time on wavefront system    -   2. A dilated pupil at measurement time on wavefront system,        using multiple measurements    -   3. A dilated pupil on a surgical system following formation of        the flap

To align at least any two images from a mathematics point of view, it isassumed that there is enough information in each of the images to allowfor the precise computation of the translational and rotational offsetsbetween pairs of images such that any two images, by way of example, maybe overlaid with acceptably small errors. This condition satisfied, anoptimized linear transformation between these image pairs is determined.The transformation is uniquely determined by three parameters: atranslation vector r₀=(a, b) (a and b are the x and y coordinates of thetranslation, respectively) and a rotation angle θ.

Image Processing

The Gauss filter is used to eliminate the noise of both images and isdefined as:G(x, y, σ ₁, σ₂)=g(u(x, y),σ₁)·g′ _(ν)(ν(x, y),σ₂)  1where $\begin{matrix}{{{g\left( {u,\sigma} \right)} = {\frac{1}{\sqrt{2\quad{\pi\sigma}^{2}}}{\exp\left( {- \frac{u^{2}}{2\quad\sigma}} \right)}}};{{g_{v}^{\prime}\left( {v,\sigma} \right)} = {{- \frac{v}{\sigma}}{g\left( {v,\sigma} \right)}}}} & 2\end{matrix}$andu(x, y)=cos θ·x−sin θ·y  3ν(x, y)=sin θ·x+cos θ·y  4is the rotation of the point (x, y) and θ is the angle of rotation. Hereθ is set to be either 0 or Π/2, which means the filter will eliminatethe noise either in the x direction or the y direction. The standarddeviation (σ) determines the shape of the filter.

Let Im(x, y) represent the image data function. Applying the Gaussfilter to the image function is equivalent to making the convolution ofthese two functions.NewIm(x, y)=Im(x, y)*G(x, y,σ ₁,σ₂)  5

Next the threshold ξ is computed.

 ξ=λ·max|NewIm(x, y)|+(1−λ)·min|NewIm(x, y)|  6

where 0<λ<1. The threshold to the new image file is applied as$\begin{matrix}{{{{Im}N}\left( {x,y} \right)} = \left\{ \quad\begin{matrix}{{{{{{New}{Im}}\left( {x,y} \right)}}\quad{if}\quad{{{{New}{Im}}\left( {x,y} \right)}}} > \xi} \\{\xi\quad{otherwise}}\end{matrix}\quad \right.} & 7\end{matrix}$

A bilinear interpolation method is used to determine the edge point, thefollowing comprising a thin function:P=(1−α)[(1−β)P ₀ +βP ₂]+α[(1−β)P ₁ +βP ₃]  8where gradient vectorgradient of Im(x, y)=(α,β)  9and P_(i) are points in a neighborhood of (x, y).Image Mapping

After processing both images, the best parameters in this lineartransformation should be found. The “best” means that, in a givenparameter space, it is desired to find a point (parameters) in thatspace, such that under these parameters the linear transformationminimizes the error between those pairs of images.

The linear transformation is defined as: $\begin{matrix}{\left( \quad\begin{matrix}x^{\prime} \\y^{\prime}\end{matrix}\quad \right) = {{\left( \quad\begin{matrix}{\cos\quad\theta} & {{- \sin}\quad\theta} \\{\sin\quad\theta} & {\cos\quad\theta}\end{matrix}\quad \right)\left( \quad\begin{matrix}{x - {center}_{x}} \\{y - {center}_{y}}\end{matrix}\quad \right)} + \left( \quad\begin{matrix}a \\b\end{matrix}\quad \right)}} & 10\end{matrix}$

The criterion to find the best transform parameters is to minimize theerror: $\begin{matrix}{\min\limits_{{{({a,b,\theta})}ɛ\quad D})}{\sum\limits_{({x,y})}{{{{{Im}N}_{prior}\left( {x,y} \right)} - {{{Im}N}_{post}\left( {x^{\prime},y^{\prime}} \right)}}}}} & 11\end{matrix}$The pair (center_(x), center_(y)) is the coordinate of the center pointof the limbus from one image.D={(a, b, θ)|a ₁ <a<a ₂ , b ₁ <b<b ₂, θ₁<θ<θ₂}  12is the parameter (searching) space. The problem is to determine thecenter-point coordinate (center_(x), center_(y)) and the searching space{a₁,a₂,b₁,b₂,θ₁,θ₂}. The limbus is manually located in this embodimenton both images to obtain the center coordinate (center_(x), center_(y))from the measurement system, and the center coordinate (center_(xx),centers_(yy)) from the surgical system. The search region is defined asa ₁=center_(xx) −k, a ₂=center_(xx) +k  13b ₁=center_(yy) −k, b ₂=center_(yy) +k  14where k is a integer. The searching resolution is Δθ=0.5°, and thesearch range is ±25°; so θ₁=−25°, θ₂=+25°. The summation Σ is taken overa reference area (x, y)εΩ. The reference area is manually located tosatisfy the assumption mentioned above.

A second embodiment of the present invention includes an orientationsystem 40 for eye surgery for correcting at least astigmatism, which isshown schematically in FIG. 9. The graphical user interface and elementsthereof are illustrated in FIG. 10. A means for making two alignmentmarks 41,42 on an eye 43 of a patient (FIG. 10) with the patient in afirst position may comprise, for example, an ink pen 44, although thisis not intended as a limitation, and alternative marking means known inthe art may also be used. In current use, the first position typicallycomprises a seated upright position. In a preferred embodiment, thealignment marks 41,42 are made at the “3 o'clock” and “9 o'clock”positions to the eye's sclera 45 just outside the margin of the limbus46. In other words, the alignment marks 41,42 are made at approximatelythe Π/2 and 3Π/2 radial positions relative to the limbus 46, with a 0radial position comprising a top point of the limbus 46. Thus thealignment marks 41,42 are made substantially collinear with a diameterof the limbus 46. One of skill in the art will recognize that thesevalues are exemplary only, and that other locations can be used withsimilar results.

A camera, preferably a color video camera 47, is provided for imagingthe eye with the patient in a second position different from the firstposition. Typically the second position comprises a prone position.

The system 40 (FIG. 9) also comprises a computer 48 that has input andoutput means. One input 49 is in electronic connection with the camera47. Means are also in communication with the computer's input and outputmeans for displaying the eye image to an operator via, for example, agraphical user interface (FIG. 10). Display hardware may comprise, forexample, a color video display monitor 50. An operator input device,which may comprise, for example, a mouse 51, is also in electroniccommunication with another input 52 to the computer 48. Alternatively,other operator input devices may be contemplated; for example, themonitor 50 may comprise a touch screen.

In a preferred embodiment, a corrective system 53 to be used inperforming surgery, for example, laser ablation surgery on the cornea,comprises an eye tracker 54 as known in the art. In this embodiment, themonitor 50 displays both a tracked eye image 55 and an untracked eyeimage 56 (FIG. 10).

A first software routine 57 is resident in the computer 48 for routingthe eye images to the monitor 50 and also for superimposing a graphicalreticle 58 onto the tracked eye image 55. The first software routine 57further permits the graphical reticle 58 to be moved by the operatorunder control of the mouse 51. The graphical reticle 58 comprises acircle 59 for superimposing on the eye's limbus 46 and a cross-hairincluding a pair of perpendicular lines 60,61, both of which aresubstantially diametric with the circle 59. Typically the generallyhorizontal line 60 is used to align with the alignment marks 41,42 onthe eye 43. In a color system, the graphical reticle 58 comprises acolor for contrasting with the eye 43, such as, but not limited to,yellow.

The monitor 50 preferably is adapted to display a graphical userinterface 62 (FIG. 10) that has an interactive control sector 63thereon. It will be understood by one of skill in the art that thisgraphical user interface is exemplary only, and that any number of suchinterfaces may be envisioned so long as the ability to align reticles isprovided.

As shown in the exemplary screen of FIG. 10, the control sector 63comprises a plurality of control sectors, in the form of “buttons,” theactivation of which moves the graphical reticle 58 in a desireddirection. Here the buttons comprise two for horizontal movement, “left”64 and “right” 65, two for vertical movement, “up” 66 and “down” 67, andtwo for rotation, counterclockwise 68 and clockwise 69. Clicking onthese buttons 64-69 with the mouse 51 causes motion of the graphicalreticle 58 on the interface 62 in the indicated direction, as mediatedby the first software routine 57 (see rotated graphical reticle in FIG.10).

In addition, a button 72 performs recentering of the lines 60,61 overthe cornea.

A second software routine 71 is also resident in the computer 48 forcalculating an orientational change to be applied to a correctivesurgical procedure. The procedure, also resident in the computer 48, isto be performed on the eye 43 with the patient in the second position.Such a procedure may comprise, for example, an ablation correctionprofile that had been determined by a measurement system 71 inelectronic communication with the computer 48, with the patient in thefirst position.

It will be understood based on the teachings of the present inventionthat in addition to images viewed on the surface of the eye, theposition of the retina and any movement thereof may be determined usingthe above methods to view images on the retina. By way of example, thevideo camera may be replaced by a scanning laser ophthalmoscope, asdisclosed in U.S. Pat. No. 6,186,628 to Van de Velde, which disclosureis hereby incorporated by reference; a retinal nerve fiber layeranalyzer, as disclosed in U.S. Pat. No. 5,303,709 to Dreher et al.,which disclosure is hereby incorporated by reference; or a fundus camerato provide images of blood vessel patterns that can be used in the samemanner as scleral blood vessels as herein described.

In the foregoing description, certain terms have been used for brevity,clarity, and understanding, but no unnecessary limitations are to beimplied therefrom beyond the requirements of the prior art, because suchwords are used for description purposes herein and are intended to bebroadly construed. Moreover, the embodiments of the apparatusillustrated and described herein are by way of example, and the scope ofthe invention is not limited to the exact details of construction.

1. An orientation system for corrective eye surgery comprising: means for performing a first image mapping an eye of a patient in a first position using a predetermined eye feature; means for filtering the first image map to reduce noise; means for processing the filtered first image map to determine an edge location of the feature in two dimensions; means for performing a second image mapping of the eye of the patient in a second position different from the first position using the feature; means for processing the second image map to locate the feature; and software means for calculating an orientational change to be applied to a corrective surgical procedure to be performed on the eye with the patient in the second position, the procedure comprising a correction profile determined with the patient in the first position.
 2. The system recited in claim 1, wherein the first image mapping performing means comprises a charge-coupled-device camera having means for capturing a video image.
 3. The system recited in claim 1, wherein the first image mapping performing means comprises one of a scanning laser ophthalmoscope and a retinal nerve fiber layer analyzer.
 4. The system recited in claim 3, wherein the predetermined eye feature comprises a portion of a blood vessel in a sclera of the eye.
 5. The system recited in claim 1, wherein the filtering means comprises a Gauss filter.
 6. The system recited in claim 1, wherein the first image map processing means comprises means for defining at least one edge of the predetermined eye feature.
 7. The system recited in claim 6, wherein the defining means comprises means for defining a plurality of edge locations in two dimensions.
 8. The system recited in claim 7, wherein the first image map processing means further comprises means for providing a mapping of edge locations.
 9. The system recited in claim 8, wherein the mapping providing means comprises a thin function.
 10. The system recited in claim 1, wherein the predetermined correction profile comprises a desired corneal profile to be achieved with an excimer laser, and the orientational change calculating means comprises means for reorienting a coordinate system of the laser.
 11. An orientation system for corrective eye surgery comprising: means for performing a first image mapping an eye of a patient in a first position using a predetermined eye feature; means for filtering the first image map to reduce noise; means for processing the filtered first image map to determine an edge location of the feature in two dimensions; means for performing an objective measurement on the eye to determine a desired correction profile for improving visual acuity in the eye; means for performing a second image mapping of the eye of the patient in a second position different from the first position using the feature; means for processing the second image map to locate the feature; and software means for calculating an orientational change to be applied to the correction profile with the patient in the second position.
 12. A method for orienting a corrective program for eye surgery comprising the steps of: performing a first image mapping of an eye of a patient in a first position using a predetermined eye feature; filtering the first image map to reduce noise; processing the filtered first image map to determine an edge location of the feature in two dimensions; performing a second image mapping of the eye of the patient in a second position different from the first position using the feature; processing the second image map to locate the feature; and calculating an orientational change to be applied to a corrective prescription for a surgical procedure to be performed on the eye with the patient in the second position, the procedure comprising a correction profile determined with the patient in the first position.
 13. The method recited in claim 12, wherein the first image mapping performing step comprises capturing a video image with a charge-coupled-device camera.
 14. The method recited in claim 12, wherein the first image mapping performing step comprises capturing a video image with one of a scanning laser ophthalmoscope and a retinal nerve fiber layer analyzer.
 15. The method recited in claim 12, wherein the predetermined eye feature comprises a portion of a blood vessel in a sclera of the eye.
 16. The method recited in claim 12, wherein the filtering step comprises applying a Gauss filter on the first image map.
 17. The method recited in claim 12, wherein the first image map processing step comprises defining at least one edge of the predetermined eye feature.
 18. The method recited in claim 12, wherein the defining step comprises defining a plurality of edge locations in two dimensions.
 19. The method recited in claim 12, wherein the first image map processing step further comprises providing a mapping of edge locations.
 20. The method recited in claim 19, wherein the mapping providing step comprises applying a thin function to the first image map.
 21. The method recited in claim 12, wherein the corrective surgical procedure comprises a desired corneal profile to be achieved with an excimer laser, and the orientational change calculating step comprises reorienting a coordinate system of the laser.
 22. A method of aligning an eye, comprising the steps of: (a) obtaining a first image of an eye, the eye being in a first position; (b) locating a feature of the eye in the first image, the feature comprising one of a sclera blood vessel, a retinal blood vessel, and a retinal nerve; (c) obtaining a second image of the eye with the eye in a second position, the second position being different from the first position; (d) locating the feature of the eye in the second image; (e) comparing the location of the feature in the first position to the location of the feature in the second position; and (f) calculating a change in orientation of the eye from the first position to the second position based on the comparison of the of the location of the feature in the first position to the location of the feature in the second position.
 23. The method recited in claim 22, wherein the first image and the second image are obtained by a charge-couple-device camera.
 24. The method recited in claim 22, wherein the first image and the second image are obtained using a scanning laser ophthalmoscope.
 25. The method recited in claim 22, wherein the first image and the second image are obtained using a retinal nerve fiber layer analyzer.
 26. A method of performing laser refractive correction on an eye comprising the steps of: (a) obtaining a first image of an eye, the eye being in a first position; (b) locating a feature of the eye in the first image, the feature comprising one of a sclera blood vessel, a retinal blood vessel, and a retinal nerve; (c) calculating a laser shot pattern based on the first image of the eye; (d) obtaining a second image of the eye with the eye in a second position, the second position being different from the first position; (e) locating the feature of the eye in the second image; (f) comparing the location of the feature in the first position to the location of the feature in the second position; (g) calculating a change in orientation of the eye from the first position to the second position based on the comparison of the of the location of the feature in the first position to the location of the feature in the second position; and (h) adjusting the shot pattern of the laser based on the calculated change in orientation of the eye from the first position to the second position.
 27. The method recited in claim 26, wherein the first image and the second image are obtained by a charge-coupled-device camera.
 28. The method recited in claim 26, wherein the first image and the second image are obtained using a scanning laser ophthalmoscope.
 29. The method recited in claim 26, wherein the first image and the second image are obtained using a retinal nerve fiber layer analyzer. 